1. Field of the Invention
The present invention relates to an optical pickup apparatus for recording and/or reproducing information for use with a recording medium and, more particularly, to an optical pickup apparatus and a method for detecting a tilt amount of the recording medium.
2. Description of the Related Art
Various types of optical recording media are now under development such as a write-once DVD-R (Digital Versatile Disc—Recordable) a rewritable DVD-RW (DVD—Rewritable) and a DVD-RAM (DVD—Random Access Memory). The optical recording media have large recording capacities of, for example, several gigabytes.
An angular deviation usually occurs between the optical axis of a beam emitted from an optical pickup and the normal direction of the optical disc at an illuminated position due to warp of the optical disc when the optical disc is rotated during recording or reproduction in an optical disc recording/reproducing apparatus. The angular deviation, i.e., tilt angle, occurs mainly in a radial direction of the optical disc to produce a coma aberration in the optical system. Accordingly, the occurrence of the tilt angle produces signal deterioration such as crosstalk with the adjacent track or jitter, having adverse effects on the reproducing quality of the optical disc. Moreover, there is a need to decrease the laser wavelength and increase the numerical aperture NA of the objective lens in order to decrease the spot diameter of the laser beam, particularly in performing dense recording such as in a DVD. This causes a reduction of the margin for the tilt angle. Therefore, the increase of recording density makes it difficult to neglect the affect from the adjacent pit even if there is only a slight inclination of the optical disc, thus incurring deterioration in reproduction quality.
There is a need for a tilt signal to monitor a radial tilt amount when implementing a radial tilt-servo. However, the focus position of the optical pickup differs from a tilt position detected by the tilt sensor where providing a tilt sensor to detect a tilt amount separately from the information-recording/reproducing optical system. This results in drawbacks, such as difficulty in acquiring a correct tilt amount, increase of manufacturing cost and difficulty in size reduction. There are also various disadvantages including decreased detection accuracy resulting from aging of the tilt sensor. In an attempt to overcome such disadvantages, a variety of methods have been developed to detect a tilt amount without separately providing a tilt sensor.
Japanese Patent Application Laid-open No. H11-110769 discloses a conventional tilt servo device to detect a tilt amount without separately providing a tilt sensor. The tilt servo utilizes a difference in crosstalk amount between the adjacent tracks on the right and left of the track, thereby generating a radial tilt signal. This, however, requires a complicated circuit for detecting the crosstalk amount. Moreover, there is a need for saving the data of three rotations of the disc to the buffer to detect a crosstalk amount when using a one-beam optical pickup. There is also a necessity that the data has been already recorded in the one-beam optical pickup. Therefore, the applicability is limited only to the after-recording reproduction for a reproduction-only disc or a rewritable disc.
Japanese Patent Application Laid-open No. 2000-149298 discloses a tilt detecting method of generating a radial tilt signal by taking a difference between a DPP (Differential Push-Pull) tracking signal and a DPD (Differential Phase Detection) tracking signal. However, the groove recording on the CD-R or DVD-RW essentially requires a three-beam optical system. In addition, a pre-pit portion for indicating address information must be provided on the disc, since a DPD tracking signal should be acquired upon recording.
Furthermore, in a tilt detecting method disclosed in Japanese Patent Application Laid-open No. 2000-137923, a radial tilt signal is generated by monitoring a change amount in the signal reproduced from the same pre-pit arrays arranged to the right and left from a track center alike in the CAPA (Complementary Allocated Pit Address) of a DVD-RAM disc. However, this is applicable only for a disc wherein the same pre-pit arrays are provided. Accordingly, the method cannot be applied to a disk for reproduction only.